1. Field of the Invention
This invention relates to the use of oil-charged mill-scale. Scaling in this case is defined as the formation of oxides that occurs on the surface of metals in the solid state as a consequence of a thermal effect in an oxidizing atmosphere.
2. Background Information
On ferrous materials, oxidation begins above approximately 400 degrees Celsius. The oxidation becomes particularly severe at temperatures above 600 degrees Celsius, so that the scaling limit for industrial iron is frequently defined as 650 degrees Celsius. The mill scale is formed during the rolling of the steel, and occurs during both hot rolling and cold rolling.
Depending on the rolling process, water and oil occur along with the mill scale.
Mill scale from the production of sheet steel products contains water (<40%) and oil. These components are currently expelled in an energy-intensive process by heat treatment before the scale can be used in a shaft furnace or in converters to recover the iron it contains.
Shaft furnaces include blast furnaces and cupola furnaces. All shaft furnaces are characterized by a vertical tubular shaft.
Shaft furnaces are used for the production of pig iron by the reduction of oxide iron ore with coke. The shaft furnace is charged at its upper end with iron ore, coke and additives. This charge is called the burden. A hot blast or heated air is blown laterally into the lower portion of the furnace. The molten iron and slag that are formed flow downward along the shaft furnace.
In the uppermost portion of the furnace, the burden is first dried. During this process, hydroxide iron ore is converted to oxide iron ore. The heat of the ore increases as it travels down the shaft. The reduction of the ore begins in the temperature range of 400 degrees Celsius.
The use of mill scale in the burden provides an opportunity for the economically beneficial recycling and disposal of the mill scale. The mill scale thereby reacts like the ore.
The prior art is described in DE 4101584 A1, DE 4329630 A1 and DE 3632364 C2, among other publications.
Oil-charged mill scale would make an additional advantageous contribution to the energy balance in the shaft furnace, but the use of oil is not without problems. Compounds that pollute the environment are easily formed. Some of these compounds are already contained in the oil.
The oil components are expelled by the heat treatment of the mill scale prior to the use of the mill scale. It is thereby advantageous to burn the gases that are discharged. An appropriate booster flame is provided regularly to ensure combustion even under unfavorable conditions.
The heat treatment is simultaneously used to achieve an agglomeration of the mill scale by sintering. This treatment results in the formation of large particles that are added to the burden but do not fall through between the other components of the burden.
Converters are vessels to handle molten metals and are lined with refractories, e.g. for the conversion with air by means of air or oxygen blown up or through the molten metal, in particular for the production of steel. The best-known converters are the Bessemer converter, the Thomas converter, the LD converter and the Siemens-Martin furnace, each of which is named after the corresponding process.
In the Bessemer process, steel is produced from high-silicon pig iron in an acid lined converter. The conversion takes place in the upright converter by a hot blast which is fed in through tuyeres on the floor of the converter. The Bessemer converter is tipped into a horizontal position to add the liquid pig iron and to empty the liquid steel produced. On account of the acid lining of the converter, the removal of sulfur and phosphorus is difficult.
The Thomas process is a basic blast method for the production of steel. A converter similar to the Bessemer converter is used, although in this case it is lined with a basic lining, i.e. with dolomite refractory bricks. The basic lining makes it possible to manufacture high-grade steel from high-phosphorus pig iron.
Just as in the Bessemer process, the hot blast for the conversion is blown into the upright converter through the tuyeres that are located in the base of the converter. The carbon in the pig iron is thereby oxidized. Then the phosphorus is oxidized and is absorbed by the slag. On account of the decarburization that occurs, a recarburization takes place after the conversion, for which purpose ferro alloys are used.
In the LD process, the conversion agent is pure oxygen which is blown onto the surface of the bath by means of a lance. This process can be used to convert practically all pig irons.
In the Siemens-Martin process, steel is produced according to the open hearth process in the Siemens-Martin furnace. As a result of the preheating of the combustion air in the Siemens-Martin furnace, and optionally the preheating of the fuel gas, high furnace temperatures of 1700 degrees Celsius, for example, can be reached. The high temperature then makes additional treatments possible during the conversion.
The converters also make it possible to use scrap and theoretically mill scale, too. The advantages of the use of mill-scale are to some extent offset by the oil the mill scale contains and the potential environmental pollution it causes.